Recently, with the active development of storage batteries, robots, satellites, and the like, along with the dramatically increasing demand for portable electronic products such as laptop computers, video cameras, mobile phones, and the like, research and development for high-performance secondary batteries capable of repeatedly charging and discharging has been actively made.
Currently, nickel-cadmium batteries, nickel-metal hydride batteries, nickel-zinc batteries, lithium secondary batteries, and the like are used as commercial secondary batteries. Among them, lithium secondary batteries have little to no memory effect in comparison with nickel-based secondary batteries, and thus lithium secondary batteries are gaining a lot of attention for their advantages of free charging/discharging, low self-discharging, and high energy density.
Particularly, with the steady exhaustion of carbon energy and increasing interest in the environment, the demand for hybrid vehicles and electric vehicles is recently gradually increasing all over the world including United States, Europe, Japan, and the Republic of Korea. Hybrid vehicles and electric vehicles are supplied with power for driving the vehicles from the charging/discharging energy of battery packs. Therefore, in comparison with vehicles powered by an engine alone, they have higher fuel efficiency and can eliminate or lessen the emission of pollutants, which adds to the appeal of hybrid vehicles and electric vehicles. Accordingly, research and development for vehicle batteries essential to hybrid vehicles and electric vehicles have intensified with gaining interest.
As described above, a battery is used for various kinds of mobile devices such as a notebook or a vehicle and has a limit in service time. Therefore, it is important to check accurate information of the SOC of the battery. The SOC is useful in weighing up an available service time of the battery and therefore is considered to be very important information to a user of the corresponding device. For this reason, a general device using a battery such as a notebook, a cellular phone or a vehicle estimates SOC of the battery, checks an available service time or capacity of the battery from the estimated SOC, and provides the information to the user.
The SOC of a battery generally represents a residual capacity as a percentage of Full Charge Capacity (FCC) of the battery. In order to estimate the SOC of a battery, various methods may be used; however, the most representative method is estimating the SOC by means of current integration. In the current integration, input/output currents of the battery are accumulated and added or subtracted to/from an initial capacity to obtain the SOC.
In this current integration method, since SOC is estimated from a current measured by a current sensor installed on a charge/discharge path of the battery, accurate sensing of the current sensor is very important. However, the current sensor may cause a current offset between an actual current and a measured current value due to degradation. If such a current offset occurs, an actual SOC may be different from the estimated SOC due to a current offset value which is a difference between the measured current value and the actual current value. In particular, when SOC of a battery is estimated using an algorithm such as a Kalman filter, the difference in SOC caused by such a current offset value is continually accumulated, which gradually increases an error of the estimated SOC value. However, a conventional SOC estimation technique does not consider such a current offset when estimating SOC, which results in inaccurate estimation of SOC.
If a battery state such as SOC is not accurately estimated due to a current offset as described above, serious inconvenience or damage may be caused to a user. In particular, in case of a battery used for an electric vehicle, if SOC of the battery is estimated excessively higher than its actual value, the user may not charge the battery as the user may not be able to predict exhaustion of SOC of the battery, which may cause the battery to fully discharge while the vehicle is running. In addition, a battery fully discharging while a vehicle is running can lead to many inconveniences such as vehicles being towed or even traffic accidents.